(1) Field of the Invention
The present invention relates to curved honeycomb structural bodies formed by extruding a molding material such as a ceramic or a resin.
(2) Related Art Statement
In general, honeycomb structural bodies having a number of through holes are molded by extrusion. It is not easy for molding engineers to produce such honeycomb structural bodies by a process other than the extruding process.
Therefore, in the conventionally known honeycomb structural bodies which are molded by extruding a material such as a ceramic or a resin, straight passages (through holes) are usually formed in an extruding direction.
Although a concept (configuration) of a honeycomb structural body having a curved shape can be illustrated and represented, such a curved honeycomb structural body cannot be easily actually produced. This is clear in view of the fact that it is difficult to finely define and constitute a number of curved passages with walls having a uniform thickness.
For example, as is disclosed in Japanese Patent application Laid-open No. 52-78,965, a process is known for producing curved honeycomb structural bodies, in which lengths of passages of a die itself in an extruding section of an extruder are variable to curve the honeycomb structural body toward a side of longer straight passages having a larger flow resistance during extruding.
However, according to this curved honeycomb structural body-producing process, since the pitches of the interior partition walls of the extruding die are constant, a cross sectional area of an opening of every through hole penetrating the honeycomb structural body along a curved direction is constant.
Therefore, the curved honeycomb structural bodies produced by this method have peculiar problems which do not exist in the straight honeycomb structural bodies. That is, when a fluid is flown into the curved honeycomb structural body through all curved passages (through holes) at one end face (an end face at an inlet side), the flow rate of the fluid coming out from the other end face (an end face on an outlet side) varies depending upon locations at the end face on the outlet side. As is shown in FIG. 8, it can be understood that the flow velocity (the flow rate) of the fluid flowing through a number of the through holes 5 penetrating from the end face 2 on the inlet side to the end face 3 on the outlet side of the curved honeycomb structural body 1 is greater on the radially outer side than on the radially inner side because of inertia of the fluid.
According to the present inventors' experiments, when the curved honeycomb structural body is used as a catalyst carrier in a catalyst device, it was made clear that an efficiency for purifying exhaust gases with the catalyst decreases as compared with the conventional straight honeycomb structural bodies. For example, if the catalytic power per unit surface area of the catalyst carried on the inner walls of the through holes 5 of the curved honeycomb structural body 1 is constant (uniform) over every portion, the purifying rate for the gas passing through those curved passages in which the flow rate of the gas is relatively higher (the flow amount is greater) is smaller as compared with the purifying rate of the gas passing through those curved passages in which the flow rate of the gas is slower. Therefore, the purifying rate of the fluid passing through the curved honeycomb structural body 1 becomes non-uniform depending upon the locations at the end face on the outlet side. This is considered as one of causes to reduce the purifying efficiency.